This invention relates generally to sensors for use in gas-reactive applications and is particularly directed to the detection of oxidizing and reducing gases using changes in the thermal conductivity of a solid oxide thin film arising from oxygen partial pressure dependent changes in the defect content of the film.
Oxygen sensors are the most widely used type of gas sensor, and are employed in applications related to personal safety, industrial process control, and environmental protection, to name just a few. As one example of their widespread usage, oxygen sensors are used in all modern automobiles to provide exhaust emission feedback control, as well as to monitor the performance of components such as the fuel injection system, the exhaust gas recirculation valve, and the catalytic converter. Multiple sensors can be present in a single vehicle.
A common type of oxygen sensor operates on the principle of solid-state electrolysis. Both amperometric (based on measurement of an electrical current through an electrolyte) or potentiometric (based on measurement of a voltage) styles exist. The most common sensor design used for automotive applications is called the Heated Exhaust Gas Oxygen (HEGO) Sensor. A schematic of this type of potentiometric sensor 10 is shown in FIG. 1. A zirconia thimble 12 is used as the electrolyte, which separates the gaseous exhaust region being monitored from a sealed reference region that contains a gas with a known oxygen partial pressure (typically air). A ceramic heater 20 is disposed within and extends substantially the entire length of the zirconia thimble 12. Porous platinum coatings 14, 16 respectively disposed on the inner and outer thimble surfaces serve as inner and outer electrodes. An electromotive force (EMF) develops when the sensor 10 is exposed to an exhaust gas with a pO2 that differs from that of the reference gas due to the development of different oxygen vacancy concentrations in the electrolyte near the two electrolyte-electrode interfaces.
Undesirable features associated with the HEGO-type sensor 10 shown in FIG. 1 include (1) the need for a reference gas region; (2) the relatively complex design that includes the bulk zirconia thimble 10 (the electrolyte) with a geometry that increases production costs; (3) the need for a metallic shield 18 to prevent contact with and possible breakage of the zirconia thimble; (4) possible reproducibility issues associated with the processing and use of porous electrodes (the electrode must provide electrical continuity, but at the same time be porous to allow gas to contact the electrolyte); and (5) the relative insensitivity of the sensor to changing pO2 except very near the stoichiometric gas ratio of pCO/pO2 or pH2/pO2=2, where a large step-wise change in EMF occurs.
U.S. Pat. Nos. 5,644,068 and 5,535,614 disclose a gas sensor of the thermal conductivity type for use in the quantitative analysis of fuel vapor content of a fuel-air mixture. This approach measures the thermal conductivity of the gas being sampled for the purpose of determining a composition of the gas mixture. Because the thermal conductivity of oxygen and nitrogen gases are almost identical, this type of thermal conductivity sensor could not be used in detecting the partial pressure of a gas such as oxygen in air or in a nitrogen-containing gas mixture. U.S. Pat. No. 4,369,647 is directed to a gas leakage detector which includes a solid state gas sensor having a sintered metallic oxide block which changes its thermal conductivity by chemical absorption thereto and a wire for transforming thermal conductivity changes into an electric signal. The solid state gas sensor is described as including a non-oxidizable metal wire such as of platinum which is coated with a sintered block of metallic oxide such as of SnO2. This gas leakage detector is not disclosed as sensing a partial gas pressure from one component in a gas mixture, nor is there any suggestion that the gas leakage detector could be used for sensing oxidizing and reducing gas species. The disclosed gas leakage detector is described in terms of detecting the leakage of a gas containing methane, liquid petroleum, nitrous oxide, and fluoroethylene.
The present invention addresses the aforementioned limitations of the prior art by providing a gas sensor which detects the presence of an oxidizing or reducing gas by measurements of oxygen partial gas pressure-dependent changes in solid oxide thin film thermal conductivity. These changes in thermal conductivity arise due to changes in the defect content of the thin film in response to changes in oxygen partial pressure. Compared to the HEGO-type sensor, the inventive gas sensor (1) eliminates the need for a reference gas; (2) has a simpler design by using thin films rather than bulk machined ceramics; (3) is more damage-tolerant due to the elimination of the zirconia thimble; (4) places the oxide coating in direct contact with the environment being sensed rather than buried under a porous electrode; and (5) allows the pressure regime over which maximum sensitivity is obtained to be controlled by controlling the type(s) of cation dopants.
Accordingly, it is an object of the present invention to provide a gas sensor employing a thin film for detecting the presence of an oxidizing or reducing gas.
It is another object of the present invention to provide for the fast detection of a gas using a nanocrystalline oxide coating based upon the faster diffusion of oxygen along grain boundaries than through grain interiors as in conventional larger-grained sensor coatings.
Yet another object of the present invention is to provide for the selective detection of plural gas species using plural thin film coatings disposed on an electrical conductor and having varying permeability.
A further object of the present invention is to provide a sensor which detects the presence of an oxidizing or reducing gas by measurement of the partial gas pressure-dependent changes in the thermal conductivity of a solid oxide thin film.
The present invention contemplates a gas sensor for detecting an oxidizing or reducing gas, such as O2, CO2, CO, NH3 or H2. Monitoring of a partial pressure of a gas to be detected is accomplished by measuring the amplitude of the temperature oscillations of an oxide-thin-film-coated metallic line in response to an applied variable frequency electrical current. For a fixed input power, the magnitude of the temperature rise in the metallic line is inversely proportional to the thermal conductivity of the oxide coating. The oxide coating contains multi-valent cation species that change their valence, and thus the oxygen stoichiometry of the coating, in response to changes in the partial pressure of the detected gas. Because the thermal conductivity of the thin oxide coating is dependent on oxygen stoichiometry, the temperature rise of the metallic line depends on the partial pressure of the detected gas. Upon detection of an oxidizing gas, the thermal conductivity of the oxide thin film increases, allowing the thin film to transport more heat away from the underlying metallic line. This causes the temperature of the metallic line to decrease, which is measured as a corresponding drop in the amplitude of the voltage oscillations in the metallic line. Detection of a reducing gas leads to an opposite effect. The thermal conductivity of the oxide thin film decreases, resulting in a reduction in the amount of heat that the thin film can absorb from the metallic conductor. This causes the metallic conductor""s temperature to rise, which is measured as a rise in the amplitude of the voltage oscillations in the metallic line. Measurement of thermal conductivity, and hence the partial pressure of the detected gas, employs either a modification of the three omega (3xcfx89) technique or a new method described below. Nanocrystalline ( less than 100 nm grain size) oxide coatings yield faster sensor response times than conventional larger-grained coatings due to faster oxygen diffusion along grain boundaries rather than through grain interiors. The use of a nanoporous oxide coating further improves sensor response time and reduces sensor operating temperature. Gas species may be selectively detected through the use of additional coatings with varying permeability.